The invention relates to a subscriber line interface circuit.
Today""s subscriber line interface circuits should imitate the traditional way of feeding a telephone line from an exchange battery via a feeding resistance. Thus, this feeding resistance determines the dependence of the line current on the line voltage, i.e. the line feed characteristic of the line interface circuit.
However, different countries require different feeding resistances, which results in that today""s line interface circuits are not generally usable but have to be adapted to the requirements specified in the respective country.
For a short-circuited line and for low-resistance line loads, the line current will be high which causes a high power dissipation in the feeding resistance. To avoid this, it is known to limit the maximum value of the line current. Also, this maximum value differs from country to country and, consequently, today""s line interface circuits have to be adapted also in this respect to the requirements specified in the respective country. However, in today""s line interface circuits, it is not possible to arbitrarily limit the line current to desired values in a simple way.
When the line is open, i.e. with the handset on-hook, today""s line interface circuits have the disadvantage that the line voltage can be lower than the expected, ideal line voltage for a given feeding voltage. This can depend e.g. on the fact that the associated line may have a leakage resistance or that the device, e.g. a telephone set, connected to the line draws current from the line for such reason. The value of this open-line voltage is, however, very important for some units, e.g. so-called MTUs (Maintenance and Test Units) and certain facsimile apparatuses.
Moreover, today""s line interface circuits are not adapted to adapt their line feed characteristic to possible feeding voltage or supply voltage variations.
The object of the invention is to eliminate the above disadvantages of subscriber line interface circuits known so far.
This is attained by means of the subscriber line interface circuit according to the invention mainly in that it comprises means adapted to a line, associated with the line interface circuit, an essentially constant line current of a first predetermined value for line voltages up to a first voltage having an absolute value which, by a predetermined amount, is lower than the supply voltage of the line interface circuit, means adapted to apply to the line, a line current which is inversely proportional to the line voltage and of a value between said first predetermined value and a second predetermined, lower value for line voltage between the first voltage and a second voltage having an absolute value which, by a predetermined amount, is higher than the first voltage, means adapted to apply to the line, an essentially constant line current of the second predetermined value for line voltages between the second voltage and a third voltage having an absolute value which, by a predetermined amount, is higher than the second voltage, and means adapted to maintain the line voltage essentially constant at the third voltage for line currents of lower value than said second predetermined value.
Alternatively, the line interface circuit according to the invention comprises means adapted to apply to a line, associated with the line interface circuit, an essentially constant line current of a first predetermined value for line voltages up to a first voltage having an absolute value which, by a predetermined amount, is lower than the supply voltage of the line interface circuit, means adapted to apply to the line, a line current which is inversely proportional to the line voltage and of a value between said first predetermined value and a second predetermined, lower value for line voltages between the first voltage and a second voltage having an absolute value which, by a predetermined amount, is higher than the first voltage, and means adapted to maintain the line voltage essentially constant at the second voltage for line currents of lower value than said second predetermined value.
Hereby, the subscriber line interface circuit according to the invention will be insensitive to supply voltage variations as well as to leakage currents on the line, and will also be easily adaptable to requirements in different countries.